The present application is the National Stage under 35 U.S.C. 371 of PCT/SE98/01617, filed Sep. 11, 1998, which designated the United States and was published under PCT Article 21(2) in English.
The present invention relates to a method for manufacturing a screen plate from a thin plane plate with screen slots and a screen element comprising one or several screen plates. The screen slots are achieved by metal cutting. After this cutting the plate is formed to a screen plate of a desired shape.
To manufacture a cylindrical screen drum from a plate is described in WO 93/04797. According to this publication the starting material is a rectangular plate on one side of which a number of parallel grooves have been obtained by milling. The grooves have V-shape with a small plane area in their bottom. A number of supporting strips are then welded across the grooves. From the rear side of the plate screen slots are milled in front of the grooves. Finally the plate is bent to a drum with the V-shaped grooves turned outside. Through the bending the screen slots in the finished drum will be thinner than they were in the plate in its plane form.
The screen slots manufactured according to this publication are cut in a mechanical way. In order to obtain sufficiently thin slots the edges in the milling equipment must be sharp, which leads to frequent changes with accompanying high costs.
In U.S. Pat. No. 5,587,077 there is described another solution of the problem to obtain slot openings in a plate which is formed to a screen drum or screen plate. According to this publication the openings are cut with a water jet under high pressure. Firstly one produces a number of grooves on the side of the plate which in the produced screen plate shall be found on the inlet side. In a second moment there is obtained a number of openings or slots in the bottom of the grooves using the water jet which under high pressure is squirted against the plate. From the US publication it is seen that the formed opening expands towards the outlet opening with an angle of 4-8xc2x0.
EP 0 287 267 describes the manufacture of cylindrical screen drums where the cutting of the screen slots takes place by means of laser beams. This cutting takes place firstly when the plate has been formed into a cylinder. The laser beam is directed against the outer side of the cylinder and the formed slot obtains a diminishing cross-section with its broadest part on the outlet side of the cylinder. In the embodiments shown in the drawings the angle between the walls of the slots is relatively small.
In the description of U.S. Pat. No. 5,587,077 the drawbacks of laser cutting of screen slots are said to be that the screen slot has only a slight conical form (0-2xc2x0), that the problems with burrs on the outlet side for the laser beam are important and that the cost for cutting is high.
According to the invention it now has surprisingly been found that it is possible, using laser cutting, to obtain a screen plate with high capacity for the suspension which is to pass the screen plate. According to the method of the invention the screen slots are formed by means of a laser beam for metal cutting, at which the laser beam hits the plate from one of its sides, the inlet side, and where the focus for the beam is situated on a predetermined depth between both sides of the plate. The slot is created using at least two cutting operations, in which the direction of the laser beam forms two different angles against the main plane of the plate, at which there is obtained a screen slot the cross-section of which increases against the other side of the plate, the outlet side. In the described way screen slots with every desired form, straight, bent or wave-formed, may be obtained. The two cutting operations may be obtained with a single laser nozzle which moves to and fro or by use of two laser nozzles which have been orientated in relation to each other in a desired way. For reason of simplicity from hereon only one nozzle is mentioned. The different cutting operations may as said above take place with at least two nozzles. Of course each screen slot may be cut in three or several cutting operations if so is desired. In such a case the beam may for example firstly hit the screen plate at a right angle to the same and then in a later moment be slanted in order to cut the sloping walls of the screen slot.
According to the method of the invention the laser nozzle is with advantage arranged to take at least two positions for the cutting operation and the point in which said angles cross each other is situated in an area within or above the plate, where the upper limit is half the thickness of a plate above the upper surface of the plate and the lower limit half the plate thickness below the upper surface of the plate. The cross-section of the screen slot depends on where the beams cross each other. The desired cross-section is determined with knowledge of the application for which the screen plate shall be used.
According to a preferred embodiment the laser nozzle is arranged in such a way that the laser beam at both cuttings hits the plate along one and the same line, at which the obtained cross-section of the screen slot on one of its sides is determined by the laser beam at its first cutting while the other side of the cross-section is determined by the laser beam during the second cutting. In this case the beams cross each other at the upper surface of the plate.
According to the method of the invention one may with advantage arrange the laser nozzle in such a way that it is situated at the same distance from a line perpendicular to the plate starting from the slot opening. If the cutting is carried through in this way there is obtained a screen slot which has a center line perpendicular to the plate. If one prefers a screen slot the center line of which forms an acute or a blunt angle with the main plane of the plate the cutting takes place with laser beams, where the laser nozzle during both cuttings is situated on the same side of the line perpendicular to the plate starting from the slot opening.
Of course, a cutting may take place with the laser nozzle arranged such that the beam hits the plate perpendicularly to the plate in a first cutting operation, while the second cutting takes place with a beam which forms an acute or blunt angle against the plate. The choice of direction for the slots depends on the qualities of the suspension which is to be treated in the screen.
In order to facilitate the transport through the screen plate the walls of the screen slot is suitably cut such that the walls form an angle a extending 10xc2x0. With advantage the angle a is 10-30xc2x0.
In many applications it is suitable that the plate on its inlet side has long parallel grooves in the bottom of which the screen slots are cut. Such a corrugated surface may alternatively be obtained by welding strips of a desired shape on the inlet side of the plate.
The screen plate is with advantage produced by cutting the screen slots in a plate having a thickness of 2-8 mm. The width of the screen slots on the inlet side is 0,1-0.4 mm.
The screen plate may with advantage be ed into a cylindrical screen drum intended for a pressure screen for fiber suspensions.